Four Weddings and a Funeral (1994)
Stars: Hugh Grant, Andie MacDowell
Carrie (Andie MacDowell) has a one-night stand with Charles (Hugh Grant), and then sleeps with him again despite being engaged at the time. Bad, bad, bad girl! It is clear early on that these two must end up together, or risk being horrible people. It's OK to cheat if the person you cheat with turns out to be your true love.
That's the rule. When Charles says to Carrie, "Do you think not being married to me might maybe be something you could consider doing for the rest of your life?"—that's your cue.